Globular Cluster UVIT Legacy Survey (GlobULeS) – I. FUV–optical colour–magnitude diagrams for eight globular clusters

Abstract

We present the first results of eight globular clusters (GCs) from the AstroSat/UVIT Legacy Survey programme GlobULeS based on the observations carried out in two far-ultraviolet (FUV) filters (F148W and F169M). The FUV–optical and FUV–FUV colour–magnitude diagrams (CMDs) of GCs with the proper motion membership were constructed by combining the Ultra Violet Imaging Telescope (UVIT) data with Hubble Space Telescope (HST) UV Globular Cluster Survey data for inner regions and Gaia Early Data Release for regions outside the HST’s field. We detect sources as faint as F148W ∼ 23.5 mag, which are classified based on their locations in CMDs by overlaying stellar evolutionary models. The CMDs of eight GCs are combined with the previous UVIT studies of three GCs to create stacked FUV–optical CMDs to highlight the features/peculiarities found in the different evolutionary sequences. The FUV (F148W) detected stellar populations of 11 GCs comprise 2816 horizontal branch (HB) stars [190 extreme HB (EHB) candidates], 46 post-HB (pHB), 221 blue straggler stars (BSSs), and 107 white dwarf (WD) candidates. We note that the blue HB colour extension obtained from F148W − G colour and the number of FUV detected EHB candidates are strongly correlated with the maximum internal helium (He) variation within each GC, suggesting that the FUV–optical plane is the most sensitive to He abundance variations in the HB. We discuss the potential science cases that will be addressed using these catalogues including HB morphologies, BSSs, pHB, and WD stars.